1. Field of the Invention
The present invention relates to a method of allocating a particular magnetic tape device for a user program from among a plurality of magnetic tape devices provided in a data processing system.
2. Description of the Prior Art
The unit of work performed in a data processing system (including the execution of one or more user programs) is called a "job". The system resource such as I/O is exclusively occupied by this job unit. In the case where the use of a magnetic tape device is required, the operator enters the job by designating a magnetic tape device prior to the execution of the job. There are a few methods of designation of a magnetic tape device. In one method, a particular device is designated. The inconvenience of this method is that if the device designated is occupied (used) exclusively by another job, the allocation thereof is impossible until the occupation is cancelled and therefore the desired job cannot be executed until the cancellation of the occupying job. Another method consists in dividing magnetic tape devices into a plurality of groups, in which the operator designates one group and a device included in the group is selected by control of the data processing system.
In conventional methods of selective allocation of a device from a given device group, an unoccupied device is searched for in an ascendant or descendant order of addresses of the devices making up the group, so that a device first found unoccupied is selected and allocated. Nevertheless, the unoccupied device may be in one of several statuses. For example, the magnetic tape for another job may or may not be left loaded in the device. The execution of the next job can of course be started earlier when the magnetic tape is not loaded. Conventionally, a device is allocated regardless of the dynamic status of the device, thus making an optimum allocation impossible.